Revert "[flang] Use precompiled parsing headers (#130600)"

This reverts commit 0c5eb4d.

Author: mrkajetanp

flang/lib/Frontend/CMakeLists.txt

@@ -72,11 +72,3 @@ add_flang_library(flangFrontend
   clangBasic
   clangDriver
 )
-
-target_precompile_headers(flangFrontend PRIVATE
-  [["flang/Parser/parsing.h"]]
-  [["flang/Parser/parse-tree.h"]]
-  [["flang/Parser/dump-parse-tree.h"]]
-  [["flang/Lower/PFTBuilder.h"]]
-  [["flang/Lower/Bridge.h"]]
-)